The present invention relates to a device for controlling a power source of an electrophotographic copier, facsimile apparatus or similar image forming apparatus and, more particularly, to a controller for controlling a bias power source to vary a bias voltage to be applied from the power source to, for example, a developing roller of a developing unit in association with image density.
Two different methods are available with an image forming apparatus of the kind described for controlling a bias voltage, i.e., a method which sets a level of the bias voltage which is associated with one of a plurality of command signals each being representative of a different bias voltage level, and a method which increases the frequency of a trigger signal by a pulse width modulation (PWM) control to adjust a trigger ON time and a trigger OFF time and thereby varies the level of the bias voltage. A problem with the multiple command signal scheme is that level setting means for setting a bias voltage level is needed which results in a complicated and bulky construction. On the other hand, the PWM scheme brings about a problem that a trigger signal having such a high frequency keeps a central processing unit (CPU) of a control circuit busy and thereby imposes restriction on the controls available with the CPU. Another problem with the PWM scheme is that it needs a large-scale step-up transformer in a step-up circuit which constitutes the power source.